This invention relates to a VHF high frequency input circuit of a TV tuner.
A constitution of a VHF high frequency input circuit of a conventional TV tuner will be described with reference to FIG. 3. One end of an FM trap circuit 32 is connected to an input end S1 via a DC blocking capacitor 31.
The FM trap circuit 32 constitutes a parallel resonance circuit composed of a capacitor 32a and an inductor 32b, and resonates to a frequency of an FM broadcasting signal. An inductor 36 and an inductor 37 are connected in series to the other end of the FM trap circuit 32 via a DC blocking capacitor 33, and the other end of the inductor 37 is grounded via a DC blocking capacitor 38.
Further, an anode of a switching diode 40 is connected via a DC blocking capacitor 39 to a connecting point connecting the inductor 36 with the inductor 37. Further, one end of an inductor 43 is connected to a cathode of the diode 40 via a DC blocking capacitor 42. A cathode of a tuning varactor diode 44 is connected to the other end of the inductor 43, and an anode of the diode 44 is grounded. Further, the other end of the inductor 43 is connected to a high frequency amplifier 46 via a DC blocking capacitor 45.
In addition, a cathode of a switching diode 34 is connected to one end of the FM trap circuit 32. Further, an anode of a diode 34 is connected to one end of an inductor 41, and also is connected to a connecting point connecting the inductor 36 with the DC blocking capacitor 33. In addition, the other end of the inductor 41 is connected to the cathode of the diode 40.
Further, a terminal Fm, as for application of a bias voltage to the diodes 34 and 40, is connected to the other end of the FM trap circuit 32 via a resistor, a terminal Lb is connected to the other end of the inductor 37 via a resistor, and a terminal Hb is connected to the anode of the switching diode 40 via a resistor. In addition, a terminal Tu, as for application of a tuning voltage to the varactor diode 44 is connected to the one end of the inductor 43 via a resistor.
Next, an operation of an input tuning circuit of the conventional TV tuner will be described. When receiving a proximity TV signal of a frequency the closest to an FM broadcasting band, the switching diode is brought into a nonconductive state by respectively imparting voltages Vlo and Vfm (Vfm greater than Vlo) to the terminals Lb and Fm and by applying a reverse bias voltage to the switching diode 34, and then an FM broadcasting signal is attenuated by making the FM trap circuit 32 operable. Simultaneously, the switching diode 40 is also brought into a nonconductive state by grounding the terminal Hb and by applying a reverse bias voltage to the diode 40. Further, a voltage Vtu is applied to the varactor diode 44 from the terminal Tu. With such operations, an output signal of the received proximity TV signal is transmitted to a high frequency amplifier 46 by tuning in to a frequency of a received signal by means of an inductance composed of the inductor 36 and the inductor 37 connected to each other in series and of the inductor 41 and the inductor 43 connected to each other in series and by means of a capacitance of the varactor diode 44.
Further, when receiving a VHF low band except the proximity TV signal of a frequency the closest to the FM broadcasting band, the switching diode 34 is brought into a conductive state as a by-pass route of the FM trap circuit 32 by grounding the terminal Fm and by adding a bias voltage Vlo to the terminal Lb. Simultaneously, the switching diode 40 is brought into a nonconductive state by grounding the terminal Hb and by applying a reverse bias voltage to the switching diode 40. With such operations, similarly as described above, the output signal of the received VHF low band can be transmitted to the high frequency amplifier 46 by tuning in to a frequency of a received signal by means of an inductance composed of the inductor 36 and the inductor 37 connected to each other in series and of the inductor 41 and the inductor 43 connected to each other in series and by means of a capacitance of the varactor diode 44.
Further, when receiving a VHF high band, the switching diode 34 is brought into a conductive state as a by-pass route of the FM trap circuit 32 by grounding the terminal Fm and by adding a bias voltage Vhi to the terminal Hb. Simultaneously, the switching diode 40 is brought into a conductive state by grounding the terminal Lb and by applying a bias voltage to the diode 40. With such operations, the output signal of the received VHF high band is transmitted to the high frequency amplifier 46 by tuning in to a frequency of a received signal by means of an inductance composed of the inductor 37 and the inductor 43 and by means of a capacitance of the varactor diode 44.
In the conventional TV tuner as described above, the diode 34, the capacitor 33, the inductor 36, and the inductor 41 are connected to one another. However, these connections are conducted by means of a conductor land provided on the printed circuit board. Further, since the size of the conductor land is large, an inductance thereof is not negligible. An equivalent circuit of a portion of the conductor land paid attention to a side of the FM trap circuit 32 when receiving a VHF high band is illustrated in FIG. 4. That is, an inductor 47 having a very small inductance by means of the conductor land described above is intervened between the anode of the diode 34 and the other end of the capacitor 33. Therefore, drawbacks as described hereinafter will be generated.
When receiving the VHF high band, as described above, the switching diode 34 is brought into a conductive state without transmitting a received signal to the FM trap circuit 32. However, at this time, the inductor 47 having a very small inductance is connected to the diode 34 in series. Accordingly, the inductor 47 is connected to the FM trap circuit 32 in parallel. In the mean time, since a capacitance value of the capacitor 33 is remarkably large compared with a capacitance value of the capacitor 32a, the capacitor 33 is negligible. Further, since an inductance of the inductor 32b is exceedingly large compared with an inductance of the inductor 47, the inductor 32b is negligible.
Accordingly, a parallel resonance circuit composed of the capacitor 32a and the inductor 47 is constituted from the result described above. Further, a resonance frequency of a parallel resonance circuit by means of a capacitance of the capacitor 32a set for an FM trap frequency and a very small inductance of the inductor 47 resides in a receiving VHF high band frequency band. Therefore, a received VHF high band signal receives attenuation in the vicinity of this resonance frequency, therefore has worsened characteristics such as NF, input sensitivity, and S/N.
Thus, an object of the present invention is to provide a TV tuner in which when receiving a VHF high band, a received VHF high band signal receives no attenuation at all and no influence is exerted on characteristics such as NF, input sensitivity, and S/N.
An input circuit of a TV tuner according to the present invention includes a parallel resonance circuit composed of a capacitor and a first inductor with one end of the parallel resonance circuit being connected to an input end, an input tuning circuit connected to the other end of the parallel resonance circuit described above and capable of tuning in to frequencies of respective bands by being switched to a low band and a high band of VHF bands, and a varactor diode brought into a nonconductive state when receiving a proximity TV signal of a frequency the closest to an FM broadcasting band and brought into a conductive state when receiving other TV signals. One end of the varactor diode is connected to one end of the parallel resonance circuit described above. The other end of the varactor diode is connected to the other end of the parallel resonance circuit described above via a second inductor having a very small inductance. A resonance frequency of a parallel connecting circuit connecting in parallel the varactor diode in a nonconductive state with the parallel resonance circuit described above is set to a frequency of the FM broadcasting band and a resonance frequency of a parallel connecting circuit connecting in parallel the second inductor in a conductive state of the varactor diode described above with the parallel resonance circuit is set to a frequency higher than the high band bandwidth described above.
The input tuning circuit described above has two low band tuning inductors of the parallel resonance circuit and the varactor diode, and the two low band tuning inductors are arranged on a printed circuit board. A conductor land for connecting the other end of the parallel resonance circuit and the other end of the varactor diode with respective side ends of each of the two low band tuning inductors is provided on the printed circuit board described above. The two low band tuning inductors is connected in series by means of the conductor land, and the second inductor is constituted by means of the conductor land described above.
The input circuit of a TV tuner further includes a switching diode connected between both ends of the two low band tuning inductors connected in series described above, a first terminal connected in a DC way to a cathode of the varactor diode, a second terminal connected in a DC way to an anode of the varactor diode and a cathode of the switching diode, and a third terminal connected in a DC way to an anode of the switching diode; Wherein respective receiving bands are capable of being switched by applying bias voltages respectively to the first terminal and the second terminal and by grounding the third terminal when receiving the proximity TV signal described above, by applying the bias voltage to the second terminal and by grounding the first terminal and the third terminal when receiving a low band TV signal except the proximal TV signal described above, and by applying a bias voltage to the third terminal and by grounding the first terminal and the second terminal when receiving a high band TV signal.